If you have obtained the Silver Scarf. Then this fight shouldn't be a big problem. Any damage she throws at you gets healed off when her attack ends. You don't even need to enable easy mode to beat her!
Obviously. It's faster to use the confusion strat to finish Martlet ASAP. But for the sake of my mission (to do as much as possible in UTY blindfolded). I will chart out all her attacks
Weapon: Doesn't Matter
Accessory: Feather
Easy Mode: Disabled
Healing Items:None
Mercy > Spare
Mercy > Spare
Mercy > Spare
Mercy > Spare
When you hear the feathers about to shoot, move Up/Down and alternate between directions each shot.
Note that two feathers may attack at the same time which might trick you. But just ignore the second feather
Easy No-Hit
Mercy > Spare
Hold Bottom-Left
6 Hits Max. Chance for less
Mercy > Spare
For each feather that is about to be shot. Hold Up/Down and alternate between directions each shot.
It's like the First Attack but a bit RNG dependant since some feathers may be positioned in such a way such that moving doesn't get you a No-Hit
Easy No-Hit. Chance for more
Mercy > Spare
Hold Bottom-Left. Switch directions in a counter-clockwise rotation for each alert.
Easy No-Hit. Chance for more
Mercy > Spare
Hold Up
3 Hits Max. Chance for less
Mercy > Spare
Hold Up. Wait until Martlet flaps her wings 2 times and hold Left. When the Object hits the ground, switch moving to the opposite direction and hold Up for a while
3 Hits Max. Chance for less
Mercy > Spare
Afterwhich, Martlet realizes that you aren't attacking. And will proceed to ask to insult her
Act > Insult (Bottom-Right)
Hold Bottom-Right
2 Hits
Act > Insult
Attacks 1-7 repeat up to this point. Keep Insulting to finish the fight
Notes:
No Easy Mode, No Healing Items, Feather used